This paper sheds light on productive vocabulary development in classes of CLIL (content and language integrated learning). Participants in the study were pupils of lower-secondary school (aged 11-15) in the Czech Republic having experienced CLIL instruction in History or Civics. The investigation was aimed at measuring vocabulary acquisition gained during a year of CLIL. Half of the pupils in the experimental group already had one year of CLIL experience prior to the experiment; the second half of the pupils started with CLIL at the time of the experiment. Both of these groups were compared to one another as well as to a control group with no CLIL instruction. Data collection instruments were created on the basis of standardized Laufer & Nation's vocabulary levels tests (Laufer & Nation, 1999). As it turned out, previous experience with CLIL as well as the role of the teacher were significant factors. The study points out differences in productive vocabulary development in CLIL in terms of a one-to-two-year time frame. The role of the teacher appeared to have an undeniable importance as well. These factors are discussed in terms of language education improvement.